I. Field of the Invention
This invention relates to a process for isomerizing trichlorobenzene (hereinafter referred to as "TCB" for short). More particularly, this invention relates to a process for isomerizing TCB so as to produce 1,3,5-TCB and 1,2,3-TCB. 1,3,5-TCB and 1,2,3-TCB are useful as intermediates of pharmaceuticals and agricultural chemicals.
II. Description of the Related Art
TCB is usually produced by trichlorination of benzene. This reaction has a strong orientation so that most of the reaction product is 1,2,4-TCB. A typical reaction product of the trichlorination of benzene contains about 90% of 1,2,4-TCB, about 10% of 1,2,3-TCB and substantially no 1,3,5-TCB.
However, in equilibrated states, the concentration of 1,3,5-TCB is considerably high. G. H. Olah et al estimated the composition of the TCB isomers at 200.degree. C. as follows (J. Org. Chem. 27, 3449 (1962)):
______________________________________ 1,2,5-TCB 65% 1,2,3-TCB 4% 1,3,5-TCB 31% ______________________________________
Thus, judging from this composition in the equilibrated state, it is believed that 1,3,5-TCB can be produced by isomerizing TCB with an acid catalyst. However, the reactivity of chlorobenzene is sharply decreased as the number of substituted chlorine atom is increased so that TCB cannot be isomerized by using a usual acid catalyst.
As for 1,2,3-TCB which is also useful as an intermediate of pharmaceuticals and agricultural chemicals, although this compound is generated by the trichlorination of benzene as mentioned above, to promote the economical efficiency, it is necessary to convert 1,2,4-TCB to 1,2,3-TCB after recovering 1,2,3-TCB from the trichlorination product of benzene.
A conventional process for isomerizing TCB is known in which a mixture of TCB isomers is contacted with acid type mordenite zeolite or acid type ZSM-5 zeolite (Japanese Patent Publication (Kokoku) No. 1-9972).
However, with this conventional process, the isomerization activity of the catalyst is reduced with time. Maintaining the catalytic activity, that is, the fact that the lifetime of the catalyst is long is very important for industrial processes. Thus, the problem of the deactivation of the catalyst encountered in the conventional process must be overcome.